With development of electronic technology, many electronic devices such as mobile phone, personal digital assistant (PDA), tablet PC, netbook and ultrabook are widely used. While many electronic devices have to support AOAC (always on and always connected) mode, i.e., standby mode, in which an electronic device is active but draws lithe current. The standby time period, in another word, how long the electronic device can stay in AOAC mode is critical. As load only draws little current in AOAC mode, overall efficiency at light load is of vital importance.
Constant on-time controlled switch mode power supply is widely used in electronic devices for power conversion due to their excellent load transient response, simple internal configuration and smooth operation mode switching. An on-time period of a switch mode power supply is provided based on an input voltage VIN and an output voltage VO with traditional constant on-time control independent of load condition. Higher switching frequency Fsw is needed for faster load transient response, smaller size and weight. Take a step-down converter with 10V input voltage VIN, 1V output voltage VO and 1 MHz switching frequency Fsw as one example, on-time period TON is 100 ns within full load range, but driving loss and switching loss are dominant at light load and power loss of the step-down converter would increase with switching frequency at light load, which is not good for improvement of efficiency.